Stiff nuts



Aug. 5, 1969 A. E. JORDAN ETA!- 3,459,249

S'I'IFF NUTS Filed Jan. 10. 1968 5 Sheets-Sheet 1 INVENTORS: ArthurEdmund. Jbrdan Regihald Hugh Williams 5, 1969 A. E. JORDAN ETAL3,459,249

s'rIPF NUTS 5 Sheets-Sheet 2 Filed Jan. 10, 1968 INVENTMS:

Arum! Edmund Jordan egina/d Hug/r Williams A wzzmfifir 5, 1969 A. E.JORDAN AL 3,459,249

STIFF' NUTS med Jan. 10, 1968 s Sheets-Sheet 3 INVENTORSI v A Re g n mdl-lg tlifi sri Aug. 5, 1969 A. E. JORDAN ET AL 3,459,249

STIFF NUTS I Filed Jan. 10, 1968 5 Sheets-Sheet '4 'mvelvrons; AI-Mw-Edmunaba'qrdnn v Reginald Hug/v h ams Aug. 5, 1969 A. E. JORDAN ET3,459,249

STIFF NUTS 7 Filed Jan. 10, 1968 r 5 SheetsSheet 5 United StatesPatentOi 3,459,249 Patented Aug. 1969 Tree 3,459,249 STIFF NUTS ArthurEdmund Jordan and Reginald Hugh Williams, Birmingham, England, assignorsto G.K.N. Screws 8: Fasteners Limited, Smethwick, Warley,Worcestershire, England, a British company Filed Jan. 10, 1968, Ser. No.696,827 Claims priority, application Great Britain, Jan. 10, 1967,1,230/ 67 Int. Cl. F16b 39/02, 39/28 US. Cl. 151-21 3 Claims ABSTRACT OFTHE DISCLOSURE A stiff nut which has the required prevailing torquecharacteristics but which will avoid damage to a bolt. The stiff nut hasa collar which is closed to elliptical shape-by pressure applied at fourpoints arranged symmetrically with respect to a diameter of the nut suchthat the angle bet-ween two adjacent points on opposite sides of thediameter is between 110 and 130.

Background of the invention Field of the inventi0n.This inventionrelates to stiff nuts. The stiff nuts with which the invention isconcerned are of the type in which the bore of the nut is deformed toincrease the frictional forces which are generated between the matingthreads of the nut and of an appropriate male threaded member or bolt onwhich the nut is engaged.

In a threaded assembly comprising a male threaded member and a plainnut, the latter can only maintain a given position on the male member bybeing tightened down against a bearing surface so that the male memberis tensioned and an end face of the nut forced against the bearingsurface. Frictional forces are also present between the mating threadsof the nut and the male member but should the frictional forces betweenthe bearing surface and the end face of the nut diminish due toconditions of service (eg, vibration) the frictional forces between themating threads will also diminish and will be insufficient to preventthe nut from rotating on its male member and thus from coming loose. Astiff nut of the above type in similar conditions will resist looseningunder vibration by virtue of the increased frictional forces between themating threads due to the deformation of the bore of the nut and willthus maintain its position.

There are some generally accepted criteria for stiff nuts although thesecriteria have not yet been embodied in a universally acceptablestandard. There are three criteria. Firstly, on first application of thenut to the male memher, the nut must be capable of being turned on thefree threads of the male member by the application of a torque whichdoes not exceed a specified maximum value. Secondly, the torque requiredto remove the nut from the member after a repeated number of successiveapplications in which a proportion of the proof load of the bolt,normally 75%, is induced, must be above a specified minimum value afterthe dissipation of the induced tensile load in the bolt. The maximum andminimum torques vary for different sizes of nuts but there are figureswhich are generally accepted.

The third criterion is that after the male member has been subjected tothe specified number of such applications of the nut, normally five, themale member must pass a standard gauge and must not have been damaged soseverely by the application of the nut that it will no longer pass thisgauge.

Description of the prior art.A considerable number of proposals havebeen made for stiff nuts and those which are currently in use, whilemeeting the maximum and minimum torque criteria set forth above, havedifficulty in meeting the third criterion, that they shall not undulydamage the male member or bolt.

There are many types of stiff nuts, but stiff nuts of the above type arenormally plated and then coated with lubricant which is often wax-based.The most severe conditions for meeting the third criterion under whichstiff nuts are commonly employed is where both the nut and the bolt arezinc plated. The frictional forces between the mating threads in theseconditions promote galling and damage the plating on either the nut orthe bolt or on both the nut and the bolt. In exreme cases under theseconditions, seizure can result such that further torque applied to thenut in an endeavour to move it can cause fracture of the bolt. Theseextreme cases can occur where the nut is applied to the bolt by a powerdriver which works at a high speed thus tending to generate excessiveheat between the threads of the nut and the bolt.

There are two features of the majority of stiff nuts currentlycommercially available which militate against the satisfaction of thethird criterion mentioned above. Firstly, the deformation of the threadis such that the arc of embrace of the nut on the bolt is small whichmeans that the second criterion with respect to the prevailing torque toremove the nut to be satisfied, there must be high local pressuresbeween the mating surfaces of the threads on the nut and the bolt. Byprevailing torque we mean the torque required to turn the nut on thefree threads of the bolt, as distinct from the torque required to turnthe nut when it is engaging an abutment. We are concerned with stiffnuts in which the deformation of the thread is mainly inwardly of thebore of the nut so as to distort the bore from a circular shape,although in some cases there may be some axial deformation of the threadwhich assists in providing the stiff characteristic of the nut. The areof embrace of a stiff nut, therefore, is the sum of the individual arcsof the threads which engage when a bolt of the appropriate size isinserted into the nut. It is apparent that the greater the extent ofthis are of embrace, the lower need the pressure between the matingsurfaces of the threads be along said are to provide the necesaryminimum prevailing torque and the less risk will there be of galling ordamaging the mating threads.

The second feature is concerned with the angles of divergence betweenthe periphery of a bolt and the flanks of the relieved portions of thebore. Thus there will, around the bore, be a series of arcs ofengagement where the mating threads engage, and between each adjacentpair of such arcs there will be a relieved portion where the threads arein reduced contact. The flanks of these relieved portions will beadjacent to the arcs of engagement and will diverge from the peripheryof the bolt. The greater the angle of divergence the more likelihood isthere of high local pressures occurring between the mating threads thusincreasing the risk of galling and stripping of the plating. Since themethod of manufacture of most male threaded parts involves flat-die,thread rolling and by its nature must induce ovality, it is essentialthat the angle of divergence referred to above is kept to a minimum.

It is an object of one aspect of the present invention to provide amethod of manufacturing a stiff nut which is simple and which results inthe production of a stiff nut which is an improvement over those atpresent commercially available.

Summary of the invention According to this aspect of the invention weprovide a method of manufacturing a stiff nut from a blank having awrench-engageable body part, a tubular collar part extending from oneend of the body part and of less cross-sectional area than the body partand a continuous, screw-threaded, through bore in said parts; the methodcomprising positioning the blank with its collar part between opposeddie members, relatively moving the die members together to engage anddistort the collar with flat die faces at four positions arranged two oneach side of, and symmetrically with respect to, the diameter of thebore which is parallel to the direction of relative movement of the diemembers, the angle between the lines of intersection of a planeperpendicular to the longitudinal axis of the bore and planes containingadjacent die surfaces lying on opposite sides to the diameter beingbetween 110 and 130, the die members distorting the collar part so thatportions thereof between each pair of said adjacent die surfaces movetowards said axis of the bore so that the portion of the latter in thecollar part is out of round and is of approximately elliptical shape andhas no sharp discontinuities in sections containing or perpendicular tosaid axis.

We find that by applying force to distort the bore in the collar at fourpositions spaced as described above we obtain, in the collar part, abore which is sufficiently distorted out of round to give the desiredstiff characteristic but which nevertheless is so shaped as to preventany extensive damage occurring to a bolt when the nut is used thereon.We find that between each adjacent pair of die faces on opposite sidesof said diameter, a portion of the collar wall is forced inwardly andits radius of curvature is increased. These portions, when the nut is inuse, engage the mating threads of the bolt or male member to provide thestiff characteristic of the nut. Since, however, the inward pressure onthe collar is not directly radially inward, although of course there isan inward radial component, we have found that there is no sharpdiscontinuity in the bore of the collar between the portions thereofwhich are forced inwardly and the other or relieved portions which, as aresult of the inward movement of the first mentioned parts, are movedoutwardly. As a result, the angle of divergence between the bolt and therelieved portions mentioned above is comparatively small and thus highlocal pressures are avoided when the nut is used as the mating threadsof the nut and the bolt engage primarily over the inwardly displacedportions of the collar.

We also find that by adopting the method of deformation of the collardescribed above, the total arc of embrace between the threads in thecollar and a bolt inserted into the collar is comparatively large andmay approach 180 thus allowing the inward distortion of the collarportions to be such that the pressure between mating surfaces of thethreads on the nut and bolt to be low thus to avoid damaging the boltwhilst still being able to obtain the minimum prevailing torque which isrequired to satisfy the second criterion set forth above.

As discussed in detail below, we find that by deforming the collar ofthe nut at four positions spaced as above, we obtain improved results ascompared with stiff nuts in which the collar has been indented at lessthan four positions or at more than four positions.

We have found that where the collar is indented at three positions,there is a comparatively small total arc of embrace between the indentedportions of the collar and the bolt so that the pressures between themating threads over said are of embrace have to be high to provide theminimum prevailing torque required with the result that damage to thebolt thread ensues.

We have found that where the collar of the nut is indented at sixpositions, although the total are of embrace is increased as comparedwith a nut in which the collar is indented in only three positions, thetotal arc of embrace is still less than where the nut is indented atfour positions spaced as required by the present invention and,moreover, the angles of divergence between the flanks of each relievedportion and the periphery of the bolt a e h gh resulting in high localpressures as the mat- 4 ing threads come into contact with the threadsin the collar portion in said arcs of engagement.

It is an object of another aspect of the invention to provide a stiffnut which is an improvement over those which are commercially available.

According to this aspect of the invention we provide a stiff nutcomprising a wrench-engageable body part, a tubular collar partextending from one end of the body part and of less cross-sectional areathan the body part, a continuous, screw-threaded, through bore in saidparts, four flats on the external surface of the collar, said flatsbeing arranged two on each side of, and symmetrically with respect to, adiameter of the bore so that the angle between the lines of intersectionof a plane perpendicular to the longitudinal axis of the bore and planescontaining the surfaces of two adjacent flats on opposite sides of thediameter is between and 130, first arcuate portions of the collar partbetween each said two adjacent flats being displaced inwardly relativeto second arcuate portions of the collar part between each two adjacentflats on the same side of the diameter with respect to said axis suchthat the portion of the bore in the collar part is of approximatelyelliptical shape and has no sharp discontinuities in sections containingor perpendicular to said axis, the locations of maximum inwarddisplacement of the collar lying between two adjacent flats on oppositesides of a said diameter.

The stiff characteristics of the nut can be enhanced by providing aslight axial compression of the threads and thus the die faces and theresulting flats may bainclined relative to the axis of the nut. Thuseach flat die face or each resulting flat may be inclined at an angle ofnot more than 10 to a plane parallel to the longitudinal axis of the nutand intersecting the die face or flat in a line in a plane perpendicularto said axis. The angle will preferably be 5 for larger nuts and 10 forsmaller nuts.

We prefer that the angle between said lines of intersection isapproximately Brief description of the drawings The invention will nowbe described in detail by way of example with reference to theaccompanying drawings in which:

FIGURE 1 is an elevation of a blank from which a stiff nut embodying theinvention may be made;

FIGURE 2 is a side elevation of a stiff nut embodying the invention;

FIGURE 3 is a plan view of the nut of FIGURE 2;

FIGURE 4 is a plan view showing the method of forming the stiff nut ofFIGURES 2 and 3;

FIGURE 5 is a perspective view of one of the die slides shown in FIGURE4 and showing the angles of the die faces;

FIGURE 6 is a plan view similar to FIGURE 3 but showing the locations atwhich various measurements are made as referred to below;

FIGURE 7 is a diagram obtained by projection techniques showing theshape of the deformed bore of the collar of a nut embodying theinvention;

FIGURE 8 is a diagram similar to FIGURE 7 but showing the shape of abore of the collar of a nut, the collar having been subjected topressure in three spaced apart positions; and

FIGURE 9 is a diagram similar to FIGURE 8 but in which the collar hasbeen subjected to pressure in six spaced apart positions.

Description of the preferred embodiment Referring first to FIGURES l to7, a stiff nut embodying the invention is made from a blank indicatedgenerally at 10 in FIGURE 1 having a wrench-engageable body part 11which is of conventional hexagonal shape and which will conform in sizeto one of the various standards for nuts. Extending axially from one endof the wrench-engageable body part 11 is a collar part 12 which is of adiameter slightly less than the size of the wrench-engageable portion 11across the flats and which has an axial length indicated by the line 13which is approximately 2 to 2 /2 pitches of the nut. The collar has arounded end indicated at 14 and its side wall is tapered relative to theaxis of the blank which is indicated at 15 so that the semi-apical angleof the cone of which the side wall 16 forms part is 5. The angle oftaper will depend on the size of the nut and will preferably be 5 forlarger nuts and for smaller nuts.

The blank is formed into a stiff nut by being placed between tworelatively movable die members which are moved together to deform thecollar as will now be described.

Referring to FIGURE 4, two relatively movable die members 17 are mountedfor sliding movement towards and away from ane another in directionsalong a line 18 which is a continuation of a diameter 19 of the bore ofthe blank. The die members are slidably mounted in guides 20 and aremoved towards and away from one another by means not shown. Mountedbetween two side members 21 is a conveyor indicated generally at 22, theconveyor being arranged to move nut blanks in the direction of the arrow23 between the die members 17, the spacing between the side members 21being such as to guide the nut blanks closely and to prevent them fromturning. A nut blank 24 is shown as waiting to pass between the diemembers 17, a nut blank 25 is shown as being operated upon by the diemembers and a stiff nut is indicated at 26 which has been formed by thedie members 17.

The die members are so arranged as to engage the outer wall 16 of thecollar 12 of the blank. Each die member has two flat die surfaces 27 andas indicated in FIGURE 5, these die surfaces are inclined at an angle 28of 5 to vertical planes 29 which are parallel to the axis of the nutblank and which intersect a plane perpendicular to said axis and asexemplified by the upper surfaces 30 of the die members in lines 31.

In the present instance, the axis 15 of each nut blank is arrangedvertically so that the surface 30 is a horizontal plane and in fact thesurfaces 27 are inclined at 5 to vertical planes containing the lines 31but it will be appreciated that it is not necessary for the axis 15 ofthe nut blank to be vertical as the stiff nut is being formed. Withsmaller nuts the angles 28 would be 10.

The angle 32 between the lines 31 is 120. This is the preferred anglebut the angle 32 can lie between 110 and 130". A relief portion 33 isformed in each die member to facilitate the forming of the faces 27. Itwill be seen that these faces 27 are flat and thus are able to sliderelative to the collar wall 16 as the die members 17 are moved togetherbut the die faces may if desired in their upper parts be shaped toembrace the radiused end 14 of the collar although the portions of thedie faces which engage the wall 16 of the collar will still be flat asdescribed.

It will be seen that the angle between the lines 31 is in fact the anglebetween lines in which planes containing the flat faces 27 intersectwith a plane, e.g., the upper surface 30 of the die member, which isperpendicular to the axis 15 of the blank.

The collar is deformed by moving the die members 17 together along theline 18 to provide the desired degree of deformation and the resultingshape of the bore of the collar is generally elliptical as will bedescribed below in relation to FIGURE 7. After the collar has beendeformed, the die members 17 are retracted and the conveyor 22 moves afurther nut blank, i.e., the blank 24 to a position between the diemembers and the latter are then moved together to deform the collar ofthe nut blank 24.

Referring now to FIGURE 3, it will be seen that in the finished stiffnut, the wrench-engageable portion 11 of the body part is unchanged fromthe blank but the external surface of the collar has four flats 34, 35,36 and 37 which are arranged symmetrically with respect to the diameter19 of the bore of the nut. The flats 34 and 35 which are the adjacentflats on opposite sides of the diameter 19 lie in planes which, whenprojected on a plane perpendicular to the axis 15, result in lines insaid latter plane which intersect at an angle of That is to say theflats are similar to the die surfaces 27 which form them. Similarremarks apply to the flats 36 and 37. The result of the collar closureis to move first arcuate portions 38 and 39 of the collar wall betweenthe flats 34 and 35 and 36 and 37 respectively inwardly with respect tothe axis 15 of the bore and to move second arcuate portions 40 and 41 ofthe collar outwardly relative to said axis.

FIGURE 7 shows a diagram which has been obtained by a projectiontechnique of the shape of the distorted bore of the collar after it hasbeen deformed as described above. This diagram was obtained by insertinga brass mandrel into the deformed bore of an unthreaded nut blank andthen sectioning the collar and the mandrel in a plane perpendicular tothe axis of the bore, and then polishing the section and then surfaceilluminating the section and projecting the result on a screen fromwhich the diagram was traced. The diagram of FIGURE 7 relates to a%-inCh nut blank and is fifteen times full size. The mandrel has itsouter periphery indicated by the circle 42 and the shape of the bore isindicated by the line 43. The mandrel was not an exact fit in the borebut does indicate, as will be described below, the extent of the arc ofembrace between the bore of the collar of a nut formed from the blankand a bolt on which it is threaded.

The arrows 44 and 4-5 indicate the points at which the collar wall wasengaged by the die faces 27. It will be appreciated that the firstcontact between the outer wall 16 of the collar and each die face 27will be a point and then as the deformation continues this point willgrow into a flat as the collar is deformed.

The estimated arcs of engagement between the threads of the collar andthe threads of a bolt within the collar are defined between the lines 47and 48 and between the lines 49 and 50. The angle 51 subtended by thelines 47 and 48 at the axis 15 is approximately 90 and the angle 52subtended between the lines 49 and 50 at the axis is approximately 80.It will be seen, therefore, that the total extent of the arc of embracewhich comprises the sum of two arcs of engagement indicated at 53 and 54is approximately The are 53 is that part of the line 43 between thelines 47 and 48 and the arc 54 is that part of the line 43 between thelines 49 and 50. The arcs 53 and 54 constitute first arcuate portions ofthe collar which are moved inwardly towards the axis 15 and there aresecond arcuate parts of the collar indicated at 55 and 56 which aremoved outwardly and away from the axis 15 upon deformation of thecollar. Each of these arcs 55 and 56 forms a relieved portion in whichthe threads in the bore of the collar are in reduced engagement with thethreads on the bolt and each of these relieved portions has flanks whereit joins the arcs of engagement 53 and 54. Thus the relieved portion 55has flanks 57 and 58 and, in FIGURE 7, where the relieved portion 55merges with the arc of engagement 53 there has been drawn a line 59which is tangent to the line 42 at the end of the line 47 and a furtherline 60 which is tangent to the flank 58 of the relieved portion 55. Itwill be seen that the angle be tween the lines 59 and 60 is small sothat there is no abrupt change in shape between the relieved portions 55and the arc of engagement 53 which would result in a high local pressurebeing set up as the nut is screwed onto the bolt. Similar remarks applyto the flank 57 of the relieved portion 55 and also to the flanks 59 and59a of a relieved portion 56.

It will be seen that the bore of the collar is thus distorted so thatthere is a comparatively large angle of embrace between the distortedbore of the collar and the threads of the bolt and so that the angles ofdivergence between the flanks of the relieved portions and the peripheryof the bolt, as exemplified by the angle between the lines 59 and 60,are small thus avoiding high local pressures.

FIGURE 6 shows the location at which measurements were made of theeflective thread diameters in two samples of UNF nut made according tothe invention:

It will be seen that the points of maximum inward displacement liesubstantially on the diameter 19 of the bore i.e. the diameter alongwhich the die members are moved towards one another. It is apparent thatthe points of maximum inward displacement are thus not deformed sincethey are spaced from the flats 34, 35, 36 and 37 so that these points ofmaximum inward displacement can be on smooth curves as shown in FIGURE 7and since they are not in line with flats the danger of producing highlocal pressures as has been the case in some of the stiff nuts that havebeen previously proposed, is reduced.

As mentioned above, we have found that by flatting the collar at fourpositions spaced as described we obtain better results than with using alesser or greater number of flattening positions.

Referring to FIGURE 8, this shows a diagram obtained in a manner similarto FIGURE 7 but in which the collar of the blank was indented at threepositions equiangularly spaced around the periphery of the collar.Referring to FIGURE 8, the outline of the mandrel is indicated at 61 andthe outline of the bore of the collar at 62. The collar has beendeformed at three positions as indicated by the arrows 63 which areequiangularly spaced around the axis of the bore of the collar. Thisresults in there being three arcs of engagement, each arc of engagementbeing opposite to one of the arrows 63 and the arcs of engagement beingindicated at 64, 65 and 66. The are of engagement 64 subtends an angle67 of approximately 30 at the axis 68 of the collar, the arc ofengagement 65 subtends an angle 69 of approximately 20 at said axis andthe are 66 tends an angle 70 of approximately 35 at said axis. The totalare of embrace, therefore, is approximately half of the total arc ofembrace of a nut made according to the invention and as described inrelation to FIGURE 7.

Furthermore, the angles of divergence between the flanks of the relievedportions between the arcs 64, 65 and 66 and the periphery of the mandrelas indicated by the line 61 are greater than in FIGURE 7. Thus referringto the relieved portion 71, for example, between the arcs 64 and 65, theangle between the flank 72 of this relieved portion and the adjacentportion 73 of the periphery of the mandrel is considerably greater thanthe corresponding angle in FIGURE 7. As compared with a collar stiff nuthaving three points of indentation, therefore, the invention hasadvantages in that the are of embrace is substantially doubled and inthat said angles of divergence are less so as to reduce the risk ofgalling or stripping of the threads when the nut is used on a bolt.

Referring to FIGURE 8, this shows a diagram obsimilar to FIGURES 7 and 8but for a nut blank which has had its collar closed by being indented atsix positions spaced equiangularly around the collar. The periphery ofthe mandrel is indicated at 74 and the periphery of the collar at 75.The six indenting positions are indicated by the arrows 76 and the arcsof engagement at 77, 78, 79, 80 81 and 82. With regard to the arc ofengagement 82, the mandrel was not an exact fit within the bore of thecollar so that the angle subtended by this are has been estimated.

The angle 83 subtended by the are 77 at the axis 84 is approximately 19,the angle 85 subtended by the are 78 is approximately 23. The angle 86subtended by the are 79 is approximately 26, the angle 87 subtended byth are 80 is approximately 24, the angle 88 subtended by the are 81 isapproximately 15 and the angle 89 subtended by the arc 82 isapproximately 17. It will be seen that the total are of embrace is thusapproximately 124 which is still considerably less than the arc ofembrace of a nut embodying the invention. Furthermore, the angles ofdivergence between the flanks of the relieved portions and the peripheryof the mandrel are greater than in the case either of the three pointclosure in FIGURE 8 or nuts embodying the invention as in FIGURE 7.Taking for example the relieved portion 90 between the arcs 78 and 79 itwill be seen that the flank 91 of such relieved portion makes an anglewith the adjacent portion 92 of the periphery of the mandrel which isconsiderably greater than the corresponding angles in FIGURE 7 or inFIGURE 8 so that there will be the likelihood of high local pressuresbeing set up as the nut is used.

In conclusion, therefore, it will be seen that the invention provides astiff nut, and a method of making such a nut, in which a comparativelylarge angle of embrace is obtained whereby the local pressure over thearc of engagement between the mating threads can be kept low while theminimum prevailing torque is still obtained. Moreover, the angles ofdivergence referred to above are kept small and this also reduces thetendency to local spots of high pressure. The invention has advantagesover collar stiif nuts made by indenting the collar at three points orindenting the collar at six points as has been described above and it isbelieved that by flatting the collar at four points spaced as describedand as claimed a stiff nut of particularly advantageous construction isobtained.

Various modifications may be made without departing from the scope ofthe invention. As described, the die members 17 reciprocate horizontallyi.e. in a plane perpendicular to the axis of the nut blank and the diefaces 27 are inclined at an angle of 5 to a vertical plane intersectingthe die faces in a line. It will be apparent, therefore, to thoseskilled in the art that there will be some slight axial compression ofthe threads in the collar part providing the stiff characteristic forthe nut but the main stiff characteristic will be obtained by the inwarddeformation of the collar as described above. This angle may beincreased to 10 for small nuts as described above.

It would be possible for the die slides themselves to move in planes atangles of up to 10 to the horizontal in such a case the die facesthemselves could be perpendicular to the planes of movement of the diemembers so that the collar would still be subjected to a slight axialdeformation by the die faces. More generally considered, the die membersmay move in planes which are inclined to a plane perpendicular to theaxis of the nut blank, such planes of movement of the die members beinginclined at not more than 10 to a plane perpendicular to the axis of thenut blank.

We claim:

1. A one-piece stiff nut comprising a wrench-engageable body part, atubular collar part extending from one end of the body part and of lesscross-sectional area than the body part, a continuous, screw-threaded,through bore in said parts, a total of four indentations defining flatson the external surface of the collar, the flats being arranged two oneach side of, and symmetrically with respect to, a diameter of the boreso that the angle between the lines of intersection of a planeperpendicular to the longitudinal axis of the bore and planes containingthe surfaces of two adjacent flats on opposite sides of the diameter isbetween and first arcuate portions of the collar part between each saidtwo adjacent flats on opposite sides of said diameter being displacedinwardly towards said axis as a result of the indentations which definesaid two adjacent flats relative to second arcuate portions of thecollar part between each two adjacent flats on the same side of thediameter with respect to said axis such that the portion of the bore inthe collar part is of approximately elliptical shape to provide aprevailing torque and has no shape discontinuities in sectionscontaining, or perpendicular to, said axis, said first and secondarcuate portions curving smoothly and uninterruptedly between saidflats, the locations of maximum inward displacement of the collar lyingbetween the two adjacent flats on opposite sides of said diameter.

2. A stiff nut according to claim 1 wherein the surface of each flat isinclined at an angle of not more than 10 References Cited 3/ 1960France. l/1958 Great Britain.

10 CARL W. TOMLIN, Primary Examiner R. S. BRITTS, Assistant Examiner US.Cl. X.R.

to a plane parallel to said axis and intersecting the sur- 15 10-86

